Many types of containers and cartons formed from folded paperboard are in widespread use today in the food industry. For packaging food items, such as frozen foods, one widespread practice is to form a multi-sided carton from plastic coated paperboard, fill the carton on a continuous food processing line and then to seal the carton closure flaps and/or flanges by automatic machinery. In the past, this closing function is most often accomplished by hot air activation of the plastic coating and then pressing the flaps and flanges against the side panels of the carton.
The use of thermosetting coatings, such as polyester resin or other heat resistant materials, is desirable since a carton of this type is safe to use for packaged foods, and can be placed directly in a microwave for cooking or reheating. These coatings also provide superior barrier qualities for preventing the paperboard from absorbing juices and/or grease during cooking. Many of these packaged foods are cooked while covered or semi-covered, so as to retain maximum moisture, flavor and aroma, as well as to prevent food splatter in the oven. Cooking or reheating packaged foods in a conventional oven where the temperature often exceeds 270.degree. C. (425.degree. F.) is a big advantage for these coated cartons.
Heat activated polyester resin can also be used as the adhesive for sealing two coated surfaces together, as with the other plastics. However, in practice the sealing process is more difficult than with the other most popular plastics, such as polyethylene resin. In fact, others have had only limited success in using the activated polyester film for closing cartons. This is where on the single-sided (inside) coated paperboard, the side edge extensions of the cover are sealed to the corresponding outwardly folded side flanges of the carton body. In past practice, the cover is either a hinged panel on the carton, or a separate lid merged with the carton just prior to the closing operation.
To provide the requisite adhesive properties or tackiness to the polyester resin, it must be heated to a temperature in the range of 225.degree. to 550.degree. F., and then firm pressure promptly applied to bond the surfaces together. The key advantage of this type of seal made with polyester resin is that good seal integrity is maintained at the elevated temperatures, for example, up to 230.degree. C. or 450.degree. F. High quality seals between the side extensions on the cover and the side flanges on the body of the carton can be attained so long as sufficient hot air can be blown against the coated surfaces, and sufficient time is available for pressing the surfaces together and cooling the film.
Experience teaches that the most important parameter in fulfilling these requirements is the careful control of the heating of the surfaces to be sealed. Since the temperature required for bringing the polyester resin to a tackiness where it can be used as an adhesive is relatively high, high speed operation usually must be sacrificed. Indeed, in many prior art approaches, the closing process must be intermittent, rather than continuous, as set forth in the Oxborrow U.S. Pat. No. 4,626,234, issued Dec. 2, 1986. In this patent, the four-sided cartons or trays are fed in an intermittent fashion past a fixed heating location; i.e. where hot air is blown against the flanges of the carton and the adjacent edge extensions of the cover or lid. Downstream at another station, the carton is again momentarily stopped to provide the requisite pressure to seal the surfaces together.
Because of the demand of the industry to increase the speed of the packaging process, and to prevent the closing operation from being the bottleneck, numerous attempts have been made to seal the cover or lid to the carton in a continuous, rather than in this intermittent fashion. An approach in this regard that has been attempted in the past for specifically attaching separate flat lids to a tray is shown in the U.S. Pat. No. 4,559,092 to Oakley, issued Dec. 17, 1985. In this approach, the lid is prepositioned on the tray with flanges on the sides of the lid facing the side flanges on the body of the tray. The two juxtaposed flanges are bent away from each other by plows situated along the feed path in order to attempt to form a V-shaped gap. Ideally, the hot air is blown into the open gap of the V, and the polyester resin coating is heated to the temperature necessary for sealing. Practice, however, shows that the narrow side flanges are difficult to open up and keep open in this manner due to the stiffness and inherent memory of the polyester film on the paperboard. This is especially true in the instance where printed material on the carton includes electron beam cured ink. The hard fact is that sufficient heating usually does not take place to provide a good, high integrity seal when these prior art approaches are used.
Also, this type of closing operation is slowed by a substantial degree due to the requirement for mechanical devices, such as the side plows, to remain in constant contact with the flanges. Because of the stiffness of the polyester film and the continuous side plow engagement, it is difficult to control the tracking of the carton along the feed path. The inherent springiness of the flanges acting against stationary plows along the side and other mechanical devices used to attempt to keep the V open, causes lateral shifting of the tray and lid from side to side. This substantially restricts the speed allowed, and due to misalignments adds to the rate of failure of the closing operation.
The prevalent thinking in the industry of providing mechanical devices for holding narrow flanges apart during application of heat for sealing is shown by the Reil et al. U.S. Pat. No. 3,980,515, issued Sep. 14, 1976. The requirement for holding the flanges apart inevitably leads to a stationary, rather than continuous, sealing operation.
Accordingly, improvement in the area of sealing a multi-sided carton or container having the inside coated with heat resistant film is sorely needed. The industry is seeking a way that efficient and reliable closing of a carton with either a hinged or separate cover or lid can be accomplished in a continuous, relatively high speed fashion. It has been proposed that the best way to accomplish this result is to do away with the need for stationary side plows and other holding devices, and to in some other way assure that the two surfaces to be sealed are opened up so as to be able to be subjected to the high temperature, hot air activation in a reliable fashion. To do this, a relatively radical departure from this prior practice is indicated.
It is, therefore, an object of the present invention to provide a method and related apparatus for closing a multi-sided paperboard ovenable carton through sealing of a cover to the side flanges of the body of the carton in such a manner to overcome the shortcomings and limitations of the prior art. The result of the attainment of this objective is a closure seal exhibiting excellent quality adhesive bonding, and high seal integrity that is maintained through freezing, distribution and heating even at the high temperatures found in conventional ovens.
It is another objective of the present invention to provide reliable, hot air adhesive activation for an inside coated carton by forming at least a half-V between the edge extension of the cover and the corresponding side flange of the body of the carton.
It is another and related object of the present invention to provide a method/apparatus that allows continuous adhesive activation between the edge extensions of a cover and the side flanges of the body of a carton by presenting a reliable open gap, at least a half-V, for receiving the hot air stream.
It is still another and related object of the invention to provide a method/apparatus of forming a closed carton without the need for mechanical, stationary plows and/or other devices to hold the gap open, and thereby allow the extensions/flanges to remain free during adhesive activation.
It is still another object of the present invention to provide a system for high speed closing of a carton by sealing a cover to the body of the carton wherein the side flanges of the carton are bent downwardly during feed movement with sufficient force to overcome the natural stiffness and memory of the plastic coating, thus allowing the flange to remain free and below a plane passing through the fold lines of the flanges for a time sufficient to provide the adhesive activation.
It is still another object of the present invention to provide the sealing method and related apparatus that allows the carton to be moved along a continuous feed path from the standard forming machine through a flange bending station, a product filling station, a sealing/closure station, and finally to the end of the packaging line.
It is another object of the present invention to provide the method and system for sealing a cover to the carton in a simplified, continuous manner wherein a die with an open cavity carries a carton along the feed path, and forming blades on the die mate with platen members to mechanically bend the side flanges of the carton to the degree necessary to assure maintenance of an open half-V allowing hot air to contact the mating board surfaces and providing full adhesive activation.
Additional objects, advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.